This invention relates to a method of manufacturing a multilayered thermoplastic film having a high distinctness of image and high gloss finish. These high gloss thermoplastic films are useful as computer graphic films.
It is often desirable to use polymer films in computer graphics applications that have a high gloss finish. Such polymer films are currently produced by a number of different manufacturing methods.
Polymer films typically used with graphic applications are calendered. The calendaring operation compresses the film between a series of steel rolls. As the calendaring increases, the film becomes smoother and glossier. Polymer films useful for graphic applications may also be dispersion cast polyvinyl chloride (PVC) monolayer films. While the use of these PVC films have met with success in the marketplace, they have also been found to be not entirely acceptable. Neither the films nor the processes for making such films are environmentally friendly. Many of the PVC films employ plasticizers that migrate into the adhesive and degrade the functionality of the adhesive composite. Plasticizer migration is also considered to be related to objectionable spotty film appearance, sometimes referred to as mottle. This problem is especially troublesome in warm weather climates. In addition, many of the processes for making the PVC films are costly.
Another method of making high gloss polymeric films is to dispersion cast the polymer film onto a polyester casting sheet or onto paper coated with polyester. The films produced have relatively high distinctness of image and gloss, however, the cost of the polyester casting sheet is an undesired added expense.
High gloss polymeric films may be prepared by extrusion casting the polymeric film material onto a polished casting roll and pinning the film as it exits the extrusion die to the casting roll with an air knife. There are several problems associated with this process, including the inconsistent appearance of the polymeric film. The air knife causes ripples or waves in the polymeric film and the film may appear very glossy in some areas and less glossy in others. Also, while the gloss measured in the machine direction is high, the gloss measured in the cross direction is much lower. In addition, gels, which are unmelted polymeric particles formed in the extrusion process or crosslinked particles inherent to a resin, may protrude from the polymeric film surface. These protruding gels cause difficulties in subsequent printing processes because of the uneven film surface. The present invention, which relates to a novel method of manufacturing high gloss thermoplastic films, overcomes these problems.
The present invention relates to a method of making a high gloss multilayer thermoplastic film comprising:
(a) forming a molten coextrudate of polymeric film layers, the polymeric film layers comprising:
at least one polyolefin core layer having a first side and a second side;
at least one first thermoplastic skin layer having an outer surface wherein the first thermoplastic skin layer overlies the first side of the core layer; and
at least one second thermoplastic skin layer having an outer surface wherein the second thermoplastic skin layer overlies the second side of the core layer;
wherein the composition of the core layer is different than the composition of the skin layers and the core layer and the skin layers being characterized by the absence of polyvinyl chloride;
(b) pinning said molten coextrudate onto a cooled casting roll having a surface roughness of no more than Ra=0.025 microinches with a pinning roll under sufficient pressure to produce a film having a smooth first skin layer outer surface, wherein the smooth first skin layer outer surface has a ratio of distinctness of image in the machine direction to distinctness of image in the cross direction in the range of 1:1 to 1.3:1 and has a 20xc2x0 gloss of at least 40 in both the cross direction and the machine direction.
In one aspect, the invention relates to a multilayered thermoplastic film, comprising: a thermoplastic core layer having a first side and a second side, the core layer comprising: a polyolefin having a density in the range of about 0.89 to about 0.97 grams per cubic centimeter; and a light stabilizer; at least one clear first thermoplastic skin layer having an outer surface, wherein the first thermoplastic skin layer overlies the first side of the core layer, the at least one first skin layer comprising a light stabilizer; and at least one second thermoplastic skin layer on an opposed second side of the core layer, the at least one second skin layer comprising a light stabilizer; the composition of the core layer being different than the composition of the skin layers, the core layer and the skin layers being characterized by the absence of polyvinyl chloride (PVC), and the outer surface of the first skin layer characterized by a ratio of distinctness of image in the machine direction to distinctness of image in the cross direction in the range of 1:1 to 1.3:1 and has a 20xc2x0 gloss of at least 40 in both the cross direction and the machine direction.